This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The Circadian Timing System (CTS) coordinates the temporal aspects of physiology and behavior in animals. Disruptions in circadian timing have adverse effects on performance and health. Adaptation to altered acceleration environments, such as space flight, results in profound changes in many physiological systems, including the CTS and sleep regulation. Understanding responses to altered gravitational environments is critical to the development of effective countermeasures for astronauts on long duration space flights and planetary expeditions, as well as for our understanding of how gravity affects humans on Earth. Acute exposure to altered acceleration and light exposure are both capable of phase-shifting the circadian clock. Since artificial gravity produced by small centrifuges is a likely countermeasure for the adverse effects of spaceflight on skeletomuscular, cardiorespiratory and other systems, the interactions of this with light countermeasures for circadian dysfunction needs to be understood. Potential light countermeasures will need to be optimized to the properties of the newly described non-image forming photoreception system consisting of intrinsically photosensitive retinal ganglion cells and projections to key areas of the brain involved in regulation of sleep-wake cycles and circadian rhythms.